The present invention relates to a process for the production of polyurethane moldings from a reaction mixture via the RIM process. The production of polyurethane moldings via the reaction injection molding (i.e. RIM) technique is well known and described in, for example, U.S. Pat. No. 4,218,543. The RIM process involves a technique of filling the mold by which highly reactive, liquid starting components are injected into the mold within a very short time by means of a high output, high pressure dosing apparatus after they have been mixed in so-called "positively controlled mixing heads".
In the production of polyurethane moldings via the RIM process, the reaction mixture generally comprises an A-side based on polyisocyanates and a B-side based on organic compounds containing isocyanate-reactive hydrogen atoms, in addition to suitable chain extenders, catalysts, blowing agents, and other additives. The polyisocyanates which are suitable for a commercial RIM process are the aromatic isocyanates, such as, for example, diphenyl methane-4,4'-diisocyanate (i.e. MDI). Various patents such as, for example, U.S. Pat. No. 4,937,366, broadly disclose aliphatic isocyanates in a long list of isocyanates which are said to be suitable for use in a RIM process. However, very few of the patents which disclose that aliphatic isocyanates are suitable for use in a RIM process have any working examples wherein an aliphatic isocyanate is used. The RIM examples of U.S. Pat. No. 4,937,366 are all based on a prepolymer of an aromatic isocyanate.
High productivity commercial RIM processes require a 30s demold time, and prefer mold temperatures less than 80.degree. C. for worker safety and energy efficiency. These are briefly described in U.S. Pat. No. 4,937,366. The RIM examples of this reference have a mold temperature of 65.degree. C. and a demold time of 35 seconds.
U.S. Pat. No. 4,772,639 describes a process for the production of polyurethane moldings reacting organic polyisocyanates with organic compounds containing isocyanate-reactive hydrogen atoms in the presence of catalysts and auxiliary agents inside a closed mold. The isocyanate component is based on (a1) mixtures of (i) 1-isocyanate-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (IPDI), and (ii) polyisocyanates containing isocyanurate groups prepared by the trimerization of a portion of the isocyanate groups of 1,6-diisocyanato-hexane, or (a2) (i) IPDI and (iii) polyisocyanates containing isocyanurate groups prepared by the trimerization of a portion of the isocyanate groups of a mixture of 1,6-diisocyanatohexane and IPDI. These reaction mixtures are broadly disclosed as being suitable for RIM processing. However, the reference requires unusually long demold times, i.e. from 3-10 minutes. These demold times are not commercially acceptable for high speed RIM production.
IPDI has NCO groups of differing reactivity due to steric hindrances. The first NCO group reacts much more quickly than the second NCO group. This fact requires higher mold temperatures and/or longer demold times for RIM systems based on IPDI. Demold times of 3-10 minutes are disclosed by U.S. Pat. No. 4,772,639.
In addition, U.S. Pat. No. 4,772,639 clearly requires that at least 10% by weight of the IPDI monomer be used. This is not attractive from a physiological viewpoint.
U.S. Pat. No. 4,642,320 discloses a process for the preparation of a molded polymer comprising reacting inside a closed mold a reaction mixture comprising (a) an active hydrogen containing material comprising a primary or secondary amine terminated polyether having an average equivalent weight of at least 500, (b) at least one chain extender, and (c) an aliphatic polyisocyanate, polyisothiocyanate, or mixture thereof, wherein the NCX index is from about 0.6 to 1.5. This process requires that component (a) have at least 25%, and preferably 50% of its active hydrogen atoms present in the form of amine hydrogens. All of the examples disclose a system based on a HDI prepolymer with amine terminated polyethers and diethyltoluene diamine at high mold temperatures and long demold times. The isocyanate used in the examples contains at least 39% by weight of HDI monomer. Although the reference discloses aliphatic isocyanates are suitable for this process, the mold temperatures are higher than normal, i.e. at least 90.degree. C., and the demold times range from about 1-5 minutes.
U.S. Pat. No. 4,764,543 discloses aliphatic RIM systems with short demold times (.about.10 seconds) and low mold temperatures (.about.70.degree. C.) that use very fast reacting aliphatic polyamines. This patent is restricted to total polyurea systems based on chain extenders which are cycloaliphatic diamines and polyethers which are amine-terminated polyethers. All of the working examples of this patent use methylene bis(4-cyclohexylisocyanate) which is difunctional, and 100% monomer. The present invention is directed to a method of obtaining fast reacting RIM systems based on urethane or urethane/urea technology with slower reacting OH terminated soft segments and extenders and slower reacting aromatic polyamine extenders. An advantage of the presently claimed systems over total polyurea systems is their improved flow characteristics during mold filling.
RIM systems are also disclosed in U.S. Pat. No. 4,269,945. These systems are based on compositions comprising a polyisocyanate, a polyol, and a specific chain extender. The specific chain extender comprises (1) at least one component selected from the group consisting of (a) a hydroxyl-containing material which is essentially free of aliphatic amine hydrogen atoms, and (b) aromatic amine-containing materials containing at least two aromatic amine hydrogen atoms and are essentially free of aliphatic amine hydrogen atoms; and (2) at least one aliphatic amine-containing material having at least one primary amine group and an average aliphatic amine hydrogen functionality of from about 2 to 16. All of the working examples in this patent use aromatic isocyanates that may be polymeric in nature. Demold times of 60 seconds are disclosed for the examples even though comparatively faster reacting aromatic isocyanates are used.
The present invention has several advantages over these references. Amine terminated polyethers (ATPE's) are comparatively expensive. Accordingly, the cost of RIM systems which are based primarily on ATPE's is significantly more than the cost of RIM systems which use conventional hydroxy group containing materials. The present systems are more economical in terms of cost as they use smaller quantities of, if any, amine-terminated polyethers. It is also possible to adjust the catalyst level of the present systems to fit the specific processing needs of various applications. These RIM systems have a wider processing window since the catalyst level allows one to tailor system reactivity for particular mold and padweight. In addition, these systems have a 30 second demold time with good greenstrength making them suitable for commercial RIM production.
In addition, certain preferred embodiments of the present invention have another advantage, i.e. isocyanates with a low monomer content are used. From an environmental and industrial hygiene viewpoint, this means lower risk is associated with the handling of the products.